The present invention relates to semiconductor devices formed of materials that make them suitable for high power, high temperature, and high frequency applications. As known to those familiar with semiconductors, materials such as silicon (Si) and gallium arsenide (GaAs) have found wide application in semiconductor devices for lower power and (in the case of Si) lower frequency applications. These semiconductor materials have failed to penetrate higher power high frequency applications to the extent desirable, however, because of their relatively small bandgaps (e.g., 1.12 eV for Si and 1.42 for GaAs at room temperature) and relatively small breakdown voltages.
Accordingly, interest in high power high temperature and high frequency applications and devices has turned to wide bandgap semiconductor materials such as silicon carbide (2.996 eV for alpha SiC at room temperature) and the Group III nitrides (e.g., 3.36 eV for gallium nitride at room temperature). These materials have higher electric field breakdown strengths and higher electron saturation velocities as compared to gallium arsenide and silicon.
Because of their wide and direct bandgap characteristics, the Group III nitride semiconductors are candidates for many applications including solar blind photodetectors, blue light emitting and laser diodes, and high temperature and high power electronics. The gallium nitride-aluminum gallium nitride (GaN/AlGaN) heterostructure has attracted special interest because of its potential applications in high mobility transistors operating at high powers and high temperatures.
In addition to other advantages, gallium nitride transistors can theoretically or in actuality demonstrate several times the power density as compared to gallium arsenide. Such higher power density permits smaller chips to handle the same amount of power which in turn provides the opportunity to reduce chip size and increasing number of chips per wafer, and thus lower the cost per chip. Alternatively, similarly sized devices can handle higher power thus providing size reduction advantages where such are desirable or necessary.
As an exemplary driving force for higher-frequency, higher-power devices, cellular telephone equipment is rapidly becoming a large market for semiconductors, potentially exceeding even that of personal computers. This increase is driving a corresponding demand for the supporting infrastructures to provide greater capabilities and performance. Expected changes include the use of higher and higher frequencies for obtaining appropriate spectrum space; e.g., from 900 MHz to higher frequencies including 2.1 GHz. Such higher frequency signals accordingly require higher power levels.
A high frequency high power device of particular interest is the high electron mobility transistor (HEMT), and related devices such as a modulation doped field effect transistor (MODFET), or a heterojunction field effect transistor (HFET). These devices offer operational advantages under a number of circumstances because a two-dimensional electron gas (2DEG) is formed at the heterojunction of two different semiconductor materials with different bandgap energies, and where the smaller bandgap material has a higher electron affinity. The 2DEG is an accumulation layer in the undoped, smaller bandgap material and can contain a very high sheet electron concentration on the order of 1012 to 1013 carriers per square centimeter (cm −2). Additionally, electrons that originate in the doped, wider-bandgap material transfer to the 2DEG, allowing a high electron mobility due to reduced ionized impurity scattering. In exemplary Group III nitride HEMTs, the two dimensional electron gas resides at the interface of a gallium nitride/aluminum gallium nitride heterostructure.
This combination of high carrier concentration and high carrier mobility gives the HEMT a very large transconductance and a strong performance advantage over metal-semiconductor field effect transistors (MESFETs) for high-frequency applications. High electron mobility transistors fabricated in the gallium nitride/aluminum gallium nitride (GaN/AlGaN) material system have the potential to generate large amounts of RF power because of their unique combination of material characteristics which includes the aforementioned high breakdown fields, wide bandgaps, large conduction band offset, and high saturated electron drift velocity.
Descriptions of recent progress in this field include, but are not limited to, U.S. Pat. Nos. 6,586,781; 6,548,333; and 6,316,793; and published applications Nos. 20020167023 and 20030102482, the contents of each of which are incorporated entirely herein by reference. Related publications include Pribble et al., Applications of SiC MESFETs and GaN HEMTs in Power Amplifier Design, International Microwave Symposium Digest, 30:1819–1822 (2002).
High power semiconducting devices of this type operate in a microwave frequency range and are used for RF communication networks and radar applications and, as noted above, offer the potential to greatly reduce the complexity and thus the cost of cellular phone base station transmitters. Other potential applications for high power microwave semiconductor devices include replacing the relatively costly tubes and transformers in conventional microwave ovens, increasing the lifetime of satellite transmitters, and improving the efficiency of personal communication system base station transmitters.
As the output power and operational frequency of these devices continue to improve, the corresponding amount of heat generated from the device, and in turn from multidevice chips and circuits, has been and will continue to increase. Additionally, the design and market-generated goals for such devices include a continued reduction in size and weight of such electronic components. Therefore, packaging density has increased and will continue to increase. As a result, some accommodation must be included to carry off excess heat or to otherwise moderate the effects of heat on operating devices.
Excessive heat can raise several problems. Conductivity decreases at higher temperatures while maximum frequency and maximum power both decrease. Higher temperatures also permit more tunneling and leaking that reduce device performance, and accelerate degradation and device failure. Stated more positively, improved thermal management can provide higher frequency operation and higher power density during a rated device lifetime.
For several crystal growth-related reasons, bulk (i.e., reasonably large size) single crystals of Group III nitrides are, for practical purposes, unavailable. Accordingly, Group III nitride devices are typically formed on other bulk substrate materials, most commonly sapphire (Al2O3) and silicon carbide (SiC). Sapphire is relatively inexpensive and widely available, but is a poor thermal conductor and therefore unsuitable for high-power operation. Additionally, in some devices, conductive substrates are preferred and sapphire lacks the capability of being conductively doped.
Silicon carbide has a better thermal conductivity than sapphire, a better lattice match with Group III nitrides (and thus encourages higher quality epilayers), and can be conductively doped, but is also much more expensive. Furthermore, although progress has been made in designing and demonstrating GaN/AlGaN HEMTs on silicon carbide (e.g., the patents and published applications cited above) a lack of consistent reliability at desired rated performance parameters continues to limit commercial development.
Accordingly, the need exists for continued improvement in high frequency high power semiconductor based microwave devices.